Advancing superconductivity for future magnets

Superconductivity has been instrumental for the realization of large particle accelerators and is a key enabling technology for a future circular proton-proton collider (FCC-hh) reaching energies of 100 TeV.

The alloy Nb-Ti is undoubtedly the most successful practical superconductor, and it has been used in all superconducting particle accelerators and detectors built to date, but the higher magnetic fields required for the High Luminosity LHC (HL-LHC) upgrade and a future circular collider (FCC) call for new materials. An enabling superconducting technology for accelerator magnets beyond 10 tesla is the niobium-tin (Nb3Sn) compound.

Nb3Sn wires suitable for producing the 11 T magnets required for the HL-LHC have been produced in industry, but the high-field magnets proposed for the FCC would require a substantial step forward in performance. In order to achieve this goal, a conductor development programme is under way at CERN.

To address the challenges of this project, a Conductor Development Workshop has bene launched by CERN. Amalia Ballarino, leader of the Superconductor and Superconducting Devices (SCD) section says: “It is the right time to create momentum for the FCC study and to bring together the current participants in our conductor development project to share recent progress and discuss future activities.”

The focus of the conductor development programme is on the development of Nb3Sn multi-filamentary wires able to meet the target non-copper critical current density (Jc) performance of 1,500 A/mm2 at 16 T and at a temperature of 4.2 K (-268.95 °C). CERN is engaged in collaborative conductor development activities with a number of industrial and academic partners to achieve these challenging goals, and the initial phase of the programme will last four years.

Presently, the conductor developed for HL-LHC reaches a performance of about 1,000–1200 A/mm2 at 16 T and 4.2 K, and a significant R&D effort is needed to increase this by 30 to 50% to meet the requirements of 16 T magnets. “The magnets for future higher energy accelerators require fundamental research on superconductors to achieve the targets in performance and cost,” says Ballarino. For the FCC magnets, thousands of tonnes of superconductor will be required. Along with an increase in performance, a more competitive cost is needed, which calls for a wire design suitable for industrial-scale production at a considerably lower cost than the state-of-the-art conductor.

Representatives from five research institutes and seven companies, from the US, Japan, Korea, Russia, China and Europe, travelled to CERN in March 2018 to attend the first Conductor Development Workshop. “Our aim is to open up a space where collaborators can discuss the current status and review different approaches to meet the target performance and cost. The meeting also serves as an invitation to potential new partners interested in joining this effort”. Two new companies attended the workshop to discuss their possible future involvement in the project, namely Luvata and Western Superconducting Technologies (WST).

The workshop started with a plenary session followed by closed meetings during which companies engaged in fruitful discussions. “Presentations in the plenary session gave a valuable overview of progress and future directions,” observed Simon Hopkins, a CERN expert on superconductivity and scientific secretary of the workshop, “but we recognise the commercial sensitivity of some of these developments. It was essential to provide an environment in which our industrial partners were free to discuss the details openly: both their proposed technical solutions and a realistic assessment of the challenges ahead.”

The early involvement of industry, and their investment in developing new technologies, is crucial for the success of the programme. One of the positive outcomes of this meeting has been that, according to Amalia Ballarino: “Thanks to their commitment to the programme, and with CERN’s support, companies are now investing in a transition to internal tin processes. It was impressive to see achievements after only one year of activity”. Several partners have produced wire with Jc performance close to or exceeding the HL-LHC specification, and all of the companies that attended the workshop had new designs to present, some of which are very innovative.

Cross-sections of prototype Nb3Sn wires developed in collaboration with CERN as part of the FCC conductor development programme.Top: optical micrographs of wires from Kiswire Advanced Technology. Bottom: electron micrographs showing a wire developed by JASTEC in collaboration with KEK. Both show the unreacted wire before the heat treatment to form the Nb3Sn compound from the niobium filaments and tin. (Credit: KAT/JASTEC. The image originally appeared in the CERN Courier, June, 2018).

The companies already producing Nb3Sn superconducting wire for the programme are Kiswire Advanced Technology Co., Ltd. (KAT); TVEL Fuel Company supported by the Bochvar Institute (JSC VNIINM); and from Japan, Furukawa Electric Co. Ltd. and Japan Superconductor Technology Inc. (JASTEC), coordinated by the Japanese High Energy Accelerator Research Organisation, KEK. Columbus Superconductor SpA will participate in the programme for other superconducting materials. Arrangements are now being finalised for Luvata and another manufacturer, Bruker EAS, to join the programme; and the participation of our Russian partner, TVEL, has been renewed.

Moreover, the organizers acknowledged the contribution of the academic partners, who are developing innovative approaches for the characterization of superconducting wires, as well as investigating new materials and processes that could help meet the required targets. Developments include the correlation of microstructures, compositional variations and superconducting properties in TU Wien; research into promising internal oxidation routes in the University of Geneva; the study of phase transformations at TU Bergakademie Freiberg; and conductors based on novel superconductors at CNR-SPIN.

Finally, during the two-day workshop a panel of experts reviewed the conductor programme and offered their invaluable insights during the last session of the workshop. Their recommendations centred on the scope and focus of the programme, encouraging an emphasis on novel approaches to achieve a breakthrough in performance, with the broadest possible participation of industrial partners, underpinned by close long-term partnerships with research institutions. “We fully share the panel’s ambition for developing novel approaches with our industrial partners,” agreed Hopkins. “Improving our understanding of the materials science of Nb3Sn wires is also essential for developing new and optimised processing methods, and we welcome the contribution of new research institutes”. A US research institute, the Applied Superconductivity Center based in the National High Magnetic Field Laboratory (Florida State University) has also joined the programme.

The structure of the FCC Conductor Development Programme, showing the activities (shaded boxes) and partners. A dotted outline and italic text indicate pending participants, whose participation is currently being finalised. (Credit: CERN)

Since the workshop, partners in the conductor development programme have continued to make good progress: the latest results will be presented at the Applied Superconductivity Conference in October 2018 (Seattle, USA), and a second edition of the workshop is planned in 2019.

We are confident that this will result in a new class of high-performance Nb3Sn material suitable not only for accelerator magnets, but also for other large-scale applications such as high field NMR and laboratory solenoids or MRI scanners for medical research.

Top image: High-performance Nb3Sn cables are being assembled by a Rutherford cabling machine in CERN's superconducting laboratory (Credits: CERN).

EASIschool '18: A summer to remember

Visiting new places, learning amazing things, experiencing different cultures and meeting interesting people from around the world: these are perhaps the right ingredients for a great summer. It’s also how one would summarize the first EASIschool that took place this summer in Vienna.

From the 30th of August to 14th of September, the MSCA H2020 EASITrain programme brought together young researchers for an intensive summer meeting! It was the culmination of the first year's activities, bringing students together from different research centres and industries for a shared experience in Vienna. The school encompassed a wide variety of engaging academic disciplines and outdoor activities.

With a comprehensive curriculum developed in coordination with all beneficiaries,, the first week offered to students the knowledge and resources to understand the inner workings of superconductivity. World-class experts covered a wide range of topics from the fundamental of superconductivity to novel characterization and manufacturing techniques, developments in high-temperature superconducting materials and possible applications outside particle physics. Students got a deep theoretical understanding and at the same time were invited to think of ways to deploy large-scale applications. Industrializing these technologies is key for future large-scale research infrastructures and could unlock their transformative potential for society.

Participants of the first EASIschool after their visit to MedAustron where they learned more about the applications that superconductivity can have outside HEP (Credit: Mattia Ortino).

The second week of the school focused on a project management training as young researchers should learn how to skillfully coordinate and manage future projects. Managing the interaction between different stakeholder groups, ensuring adequate financing and resources and conceiving a realistic timeplan along with a detailed risk analysis are among the key factors for the success of a project. Experts from TU Wien, CERN and the Economic University of Vienna discussed these aspects and offered a hand-on training to the students. In addition, during an intense one-day media training, students learned about the key concepts and methodologies of storytelling; narrating their personal stories and motivation to join EASITrain turned out to be a moving experience that brought them closer and strengthen the team spirit of EASITrainers!

EASISchool also offered a rich and diverse social programme that included a visit to the Atominstitut and MedAustron; an ion-therapy centre that exemplifies the knowledge transfer from CERN to its member states. The highlight was a public discussion on the 8th of September with the student’s participation on “Forschung? Was geht mich das an!” an event co-organized with HEPHY, the Austrian Academy of Sciences and Vienna’s Natural History museum (https://forschung.web.cern.ch/).

Alice Moros and Mattias Ortino, two of the MSCA EASITrain ERCs in front of the World Wide Würstelstand with Rolf Heuer during the public event “Research: What is there for me”? (Credit: Bill Lorenz)

Speakers included among others, Nobel Prize Winner on high-temperature superconductivity “Georg Bednorz” and CERN’s former Director General “Rolf Heuer” and Alice Moros one of the ERCs. The event coincided with the opening of the travelling photographic exhibition “CODE of the Universe” in front of the NHM as part of the “Be OPEN: Science and Society” festival in Vienna.

The travelling photographic exhibition "Code of the Universe" opened in front of Vienna's Natural History Museum.

Looking back, the first EASISchool was a big success! It offered an outstandingly diverse programme covering: the scientific foundations of superconductors, economic and technological aspects of innovation and a number of outreach and communication activities to explain the benefits of fundamental research to a wider audience. Participants learned from the experiences of their peers, absorbed the knowledge their tutors had to offer and made new long-standing friends broadening their networks! Could you think of a better way to spend your summer?

A big step towards the superconducting magnets of the future

Last April, the FRESCA2 dipole magnet reached a field of 14.6T. This field value sets a new world record for dipole magnets with a free aperture, and breaks the old record established in 2008 of 13.8T by LBNL with the HD2 dipole magnet.

The development of magnets with fields beyond 10T started in Europe in 2004 with the FP6-CARE-NED project where the basic technologies were developed and specifically the Nb3Sn conductor which is the workhorse for the HL-LHC 11 T magnet, the LHC luminosity upgrade programme and baseline option for the more powerful 16T magnets for the Future Circular Collider study.

“FRESCA2 has already played an important role in the development of the new magnets for the High Luminosity LHC and will soon help develop the next generation of magnets." says Gijs de Rijk, head of the FRESCA2 programme.

The FRESCA2 dipole magnet design and construction was started in the framework of the FP7-EuCARD-HFM project in 2009 and has been co-financed by HL-LHC. The FRESCA2 magnet is much larger than a LHC magnet, measuring 1.5 m in length and 1 m in diameter. This allows the magnet to have a large aperture, measuring 10 centimetres, so that it can house the cables being tested, as well the sensors to monitor their behaviour.

The FRESCA2 magnet before the start of the tests. (Image: Maximilien Brice/CERN).

The magnet is the outcome of a successful collaborative effort between CERN and CEA-Saclay. The technology developments for FRESCA2 were essential for the new Nb3Sn magnets of HL-LHC. Formed by the superconducting niobium-tin compound and cooled to 1.9 kelvin (-271°C), it had already reached a field of 13.3 teslas in August 2017. Then, with a modification of the mechanical pre-stressing, it started a new series of tests in April before reaching its record intensity.

FRESCA2 will also be used to test coils formed from high-temperature superconductors. The goal is to test not only the maximum electrical current but also study in depth the effects of so high magnetic fields and the behaviour of the coil. Results from these measurements feed current efforts to design high-field magnets for future energy-frontier colliders.

The magnet was tested to the nominal operating field, and achieved 13.3T in August 2017 after a very rapid training of 5 quenches. As a second step, the mechanical preload was increased and the magnet was retested in April 2018 to explore the ultimate operating limit. In this configuration FRESCA2 reached a maximum bore field of 14.6T at a temperature of 1.9K with additional 6 training quenches. The tests are currently being performed in the new purposely built test cryostat of the SM18 cryogenic test station at CERN.

This result is a major milestone in the progression towards high field accelerator magnets beyond HL-LHC. The future of FRESCA2 is to provide background fields for tests of cables and small coils, a new facility that will provide unique test capabilities.

A new step towards successful MgB2 superconducting coils

On 7th March the INFN-LASA laboratory completed the construction and successfully tested a superconducting coil in MgB2 (Magnesium di-Boride based conductor) to be used in a high order corrector magnet. Development of MgB2 coils for a Round Coil Superferric Magnet corrector was launched in the framework of the HL-LHC project IR magnets. This design, proposed in the 70s by Russian scientists, allows to create any multipole with the same round coil through a three dimensional shaping of the iron; the low curvature radius of the coil allows using MgB2 superconductor.

Application of this design to the HL-LHC high order correctors started in 2014 by our late lamented Giovanni Volpini with CERN support. Computations showed that this option had a lower efficiency in terms of longitudinal space, and therefore the classical superferric option with Nb-Ti and standard two dimensional iron shaping was retained; INFN-LASA built three correctors based on this design in the past two years, and two more types are being built in collaboration with industry. Nonetheless, the MgB2 RCSM prototype magnet has remained in the development line to explore the manufacturing aspects of this design, and to have a MgB2 corrector available for high energy accelerators, a prima for this technology.

A single coil, wound with conductor produced by Columbus Superconductors (Genova), was tested in LASA without the iron yoke; this coil is the active part of the RCSM magnet, which will be assembled and tested in September 2019. The coil, cooled at 4.2 K with liquid helium, reached without any training quench the specification current (“ultimate current”, 160 A), passing also the stability test of one hour at the ultimate current. The coil was then energized to larger currents to investigate the limiting current, which resulted in 243 A; this is 73% of the intersection of the load line with the critical current for virgin, not-degraded, conductor. It should be noted that this current limit was reached without intermediate quench (no training).

Image Credit: INFN/LASA

“The test result is beyond our expectations,” says Massimo Sorbi from INFN-LASA. “MgB2 virgin conductors are very prone to degradation when they are wound and manipulated with even better procedures used for the other common superconductor magnets based on NbTi. Our cryogenic test was also complemented with the measurement of the thermal contraction of coil (literature is lacking of this data regarding MgB2 coils), which will enable a better design of the mechanical structure for the final magnet.” “This is a relevant technological spin-off of the HL LHC project”, says Ezio Todesco, in charge of HL-LHC IR magnets, “enabled by the synergy between INFN-LASA and CERN”.